A yawing system for a wind turbine is used for controlling the position of the rotor of the wind turbine relatively to the direction of the wind. Thus, a yawing system is used for rotating a nacelle relative to a tower about a substantially vertical axis. This rotation is normally effectuated by means of one or more yaw drives. Normally, a yaw drive comprises a yaw motor and a yaw gear.
In most prior art yawing systems the motors of the yaw drives, normally designated yaw motors, are positioned in such a manner that the yaw motor protrudes from the yawing system into the nacelle. They may, for example, be mounted directly on a base plate of the nacelle. A nacelle normally contains a number of other relatively space consuming components, such as a drive train of the wind turbine, for example, including a gear and a generator, or a gearless drive train, and the space available in the nacelle for other purposes is therefore limited. As a consequence of this, yaw drives for driving the yawing system have traditionally been arranged at positions in the nacelle where other parts permit. Thus, the yaw motors have typically been positioned ‘around’ these other parts, i.e., the positions of the yaw motors, and thereby the corresponding yaw drives, have been selected from an ‘available space’ point of view, rather than with due consideration to which positions would be optimal from an operational point of view. Apart from preventing operationally optimal positioning of the yaw drives, this approach limits the freedom to design the yawing system, for example, in terms of number and positions of the drive units, as well as number and positions of other necessary parts of the yawing system, such as brake modules, various damping means, etc. This is a great disadvantage.
EP 1 571 334 discloses a wind turbine yawing system comprising a gear ring fixed to a tower and at least one geared motor fixed to a frame, meshed to the gear ring through a gear wheel. The geared motor protrudes into the nacelle as described above. The yawing system of EP 1 571 334 further comprises at least one active braking module and at least one passive braking module. The active and passive braking modules can comprise a body of equal dimensions and equal fixing means for fixing them to the frame, and one type of module can be replaced by the other. With this possibility, it is possible to vary the ratio between the number of active braking modules and the number of passive braking modules to have a braking effect adjusted to the circumstances of the wind of the place where the wind turbine is installed. Accordingly, the yawing system disclosed in EP 1 571 334 is customizable to some extent. However, the problems described above relating to the positioning of the yaw drives are not solved by this yawing system. Furthermore, it is not possible to customize the yawing force applied by the yawing motors.